i have been thinking about this for krystal (maybe in a couple of months time tho)

empty boxes from the pantry (all sizes)
paddlepop sticks
old toilet rolls/paper towel rolls
cut outs from magazines (krystals loves any pics of animals or babies/kids - you could pre-cut or tear them out, so they can stick them down)
soft drink bottles

umm there is probably heaps of other stuff, i just cant think!